High molecular weight linear polyester resins of the poly(alkylene terephthalate) family are known to be superior components in thermoplastic fibers, films and molding compositions, because of their excellent physical properties and surface appearance. The alkylene groups can have from 2 to 10 carbon atoms in the repeating units. Among the most useful such polyesters are poly(ethylene terephthalate), poly(1,3-propylene terephthalate), and poly(1,4-butylene terephthalate) resins. Because the latter crystallizes very rapidly from the melt, it can be formulated into compositions which are moldable in conventional equipment with conventional temperature and cycle times, and without the need to use nucleating agents, and thus is unique in molding compositions.
Poly(alkylene terephthalates) are commonly prepared by either one of two methods:
1. BY TRANSESTERIFICATION OF A DIALKYL TEREPHTHALATE, E.G., A (LOWER), C.sub.1 -C.sub.6 alkyl terephthalate such as dimethyl terephthalate with an excess of the corresponding alkenediol, such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,10-decanediol, and the like, followed by polymerization of the intermediate product, splitting off the excess diol at high temperature and vacuum; or
2. BY DIRECT ESTERIFICATION OF TEREPHTHALIC ACID WITH AN EXCESS OF THE CORRESPONDING ALKANEDIOL, FOLLOWED BY A SIMILAR POLYMERIZATION STEP AS IN THE FIRST METHOD.
The second method is generally recognized to provide a faster rate of polymerization, resulting in economic advantages. In the polymerization of poly(1,4-butylene terephthalate), however, the contact between the 1,4-butanediol and terephthalic acid at high temperature and in the presence of water evolved in the reaction leads to the formation of large quantities of tetrahydrofuran from butanediol as a result of a well-known acid - catalyzed dehydration reaction. A similar side reaction also interferes with the use of other alkanediols. For example, significant amounts of diethylene glycol are formed in the polymerization reaction producing poly(ethylene terephthalate). This glycol becomes incorporated in the poly)ethylene terephthalate) to the detriment of its properties.
It has now been discovered that it is possible to achieve at least partially the enhanced reaction rates obtainable with terephthalic acid and minimize its corresponding disadvantageous side-reactions, when a mixture of di(lower)alkyl terephthalate and terephthalic acid are used in the esterification reaction with an excess of the alkanediol, especially 1,4-butanediol. In this process, there is reacted a mixture of dialkyl terephthalate and terephthalic acid, containing from 5-95 mole %, preferably from 5-25 mole % terephthalic acid with an excess, e.g., as from 110 to 500 mole %, of the alkanediol, based on the mixture.
Because the side reactions are suppressed, the poly(alkylene terephthalate) resins can be obtained in a very economical fashion by the present process. Moreover, the molecular weights are surprisingly higher than would be expected from the shorter reaction times. In addition, the products may be just as easily compounded and will ultimately provide molded articles with substantially the same superior properties as those made from the best of the prior art polyesters.